The present invention generally relates to a dissociation chamber and measuring cell arrangement useful for making atomic fluorescence measurements. Volatile hydrides of a sought element are generated and flamelessly decomposed. In particular, the invention relates to such a combination wherein the dissociation chamber is heated and separated from the measuring cell.
It is known to determine elements, i.e. those which form voltatile hydrides such as selenium, by adding a reagent to a sample solution. In such an instance, the addition of the reagent causes the hydride of the sought element to be formed and drives it from the solution. This volatile hydride is usually guided by an inert gas flow into a heated measuring cell. Conventionally, the hydride is thermically decomposed in the heating measuring cell, whereby the element sought exists as free atoms. In practice, the measuring light beam of an atomic absorption spectrometer is passed through the heated measuring cell and the proportion of the sought element in the sample is determined from the degree of absorption of the measuring light beam. In an atomic absorption spectrometer the measuring light beam is usually produced by a light source the spectrum of which includes the line spectrum of the sought element.
Such an arrangement is known, for example, U.S. Pat. No. 4,208,372 issued to Bernard W. Huber on June 17, 1980.
In conventional arrangements the atomic absorption is measured in the measuring cell. As well known, the atoms in the measuring cell resonantly absorb light quanta from the measuring light beam of the atomic absorption spectrometer. The absorbed light quanta are subsequently emitted as resonance fluorescence. The fluorescence radiation so generated in equally distributed to all directions. In the direction of the measuring light beam the resonance absorption appears as an attenuation of the measuring light beam. The measurement of interest then is the degree or amount of attenuation of the intensity of the measuring light beam.
To increase the sensitivity and the accuracy of measurement it is known in the art to ignore the attenuation of the measuring light beam and observe the fluorescence radiation from a direction perpendicular to the measuring light beam. This radiation is proportional to the quantity of the sought element in the sample. A detector so positioned is not exposed to the comparatively high intensity measuring light beam but only to the comparatively lower intensity fluorescence radiation.
In one known arrangement (K. Tsujii and K. Kuga "Improvements in the Non-Dispersive Atomic Fluorescence Spectrometer Determination of Arsenic and Antimony by a Hydride Generation Technique" in Analytica Chimica Acta 97,51 to 57: 1978) hydrides of a single element sought in the sample, are guided into a flame, the hydrides are dissociated and free atoms of the sought element are formed. An excitation light beam is then directed through the flame, which beam also emerges from a light source emitting the line spectrum of the looked-for element. The fluorescence radiation is observed perpendicular to the direction of the excitation light beam by means of a photomultiplier.
This arrangement suffers from the disadvantage that the fluorescence signal carries with it a rather high noise background. Furthermore, non-specific fluorescence signals are observed, which signal appears to arise from OH-ions and thus reduce the accuracy of the measurement.
In another known arrangement (T. Nakahara, T. Tanaka and S. Musha "Flameless Atomic Fluorescence Spectrometry of Mercury by Dispersive and Non-Dispersive Systems in Combination with Cold-Vapor Technique" in Bulletin of the Chemical Society of Japan, volume 51(7) 2046 to 2051, 1978), where mercury is determined by measurement of the resonance fluorescence mercury vapor is driven off from a sample solution by a reagent. The vapor is guided through a heated measuring cell. The measuring cell in this arrangement is block shaped. An excitation light beam, containing therein the line spectrum of mercury, is directed through the measuring cell via windows in opposing sides thereof. The resultant fluorescence radiation is measured through a window which is perpendicular to the excitation light beam by use of a photomultiplier.
From a paper by T. Nakahara, T. Tanaka and S. Musha "Non-Dispersive and Dispersive Atomic Fluorescence Spectrometry of Arsenic by Utilizing the Arsine-generation Technique" in "Bulletin of the Chemical Society of Japan" volume 51(7), 2046-2051: 1978 an apparatus for the determination of arsenic is known. The arsenic is driven from the sample as volatile hydrides by the addition of reagents. This hydride is carried, via an inert gas flow, into the flame of a burner. The arsine is dissociated in the flame such that the arsenic exists in atomic form. A light beam, containing the resonance lines of arsenic, is directed through the flame into a substantially black body, i.e. a cavity, having low reflectance waves. The resonance fluorescence is again observed perpendicular to the direction of the light beam by means of a photomultiplier.